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They say police are also conducting searches in Finland, Germany and Switzerland

(CNN)At least 15 people associated with a Norway-based Kurdish terror group were arrested in a multi-nation European sweep Thursday, Italian authorities said, with officials asserting the group was helping ISIS and plotting attacks against European diplomats.

The coordinated arrests in Italy, the United Kingdom, Norway and Finland targeted alleged members of Rawti Shax, a group said to be a European offshoot of the Iraqi Kurdish jihadist network Ansar al Islam, which fought an insurgency campaign with al Qaeda in Iraq last decade.

Among those arrested was Mullah Krekar, an Ansar al Islam founder and Iraqi Kurd who lives in Norway and allegedly leads Rawti Shax from the Scandinavian nation, Italian police told reporters Thursday.

Krekar has been in and out of Norwegian prison, and his group was organizing attacks against European diplomats because of one of his most recent imprisonments -- with Krekar allegedly orchestrating the network while behind bars, Italian police said.

"The group was organizing, via (the Internet), hostile activities against diplomats from Norway and the United Kingdom, to be carried out in the Middle East" in retaliation for a recent Krekar imprisonment, said Gen. Giuseppe Governale, commander of a special operations unit of Italy's military police.

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A news release from Italian authorities went further, saying the group was plotting "to seize Norwegian diplomatic personnel present in a country in Europe or the Middle East ... in order to negotiate with the authorities for the release of Mullah Krekar in Norway."

Beyond sticking up for Krekar, the group -- having switched allegiances from al Qaeda to its rival ISIS -- primarily aims to help ISIS take over Kurdish portions of Iraq, according to Eurojust, the judicial cooperation arm of the European Union.

"(Rawti Shax) became active in providing logistical and financial support to recruiting foreign terrorist fighters to be sent to Syria and Iraq, also with the intent of training them for (a) future conflict" in Iraq's Kurdish autonomous region, Eurojust said.

According to an indictment against the 15, Italian prosecutors said one defendant is accused of calling "for violent acts such as kidnappings and attacks to be performed in Europe," and another is accused "in reference to planning of retaliatory terrorist actions to be performed in Norway."

Further details about the alleged plots weren't immediately available.

When asked for comment about the alleged plots against diplomats, the Norwegian Justice Ministry referred CNN to the Norwegian Police Security Service, or PST. That agency did not immediately respond to CNN's requests for comment.

The British Foreign and Commonwealth Office said it had no comment.

15 arrested; others sought

The arrests were directed by prosecutors in Rome, with cooperation with counterparts in other nations, after website surveillance led Italian police in 2010 to detect an alleged Italy-based Rawti Shax cell dedicated to recruiting militants, Italian authorities said.

On Thursday, seven people were arrested in Italy, four in the United Kingdom, three in Norway and one in Finland, Italian officials said. Police also searched premises in Germany and Switzerland.

All but one of those arrested are members of the Kurdish ethnic group; the other is from Kosovo, officials said.

"Some suspects could not be found, as they are believed to have traveled to the Middle East (Syria and Iraq) to join jihadist organizations," including ISIS, Eurojust said.

Italian officials said Rawti Shax essentially stems from Ansar al Islam, which fought against the new Iraqi government after a U.S.-led invasion toppled Iraqi dictator Saddam Hussein's regime in 2003.

Some Ansar members migrated to Europe after they were flushed out of Iraq, formed Rawti Shax and recently aligned with al Qaeda's rival, ISIS, Italian prosecutors said.

"(Rawti Shax's) primary objective is to violently overthrow" the leaders of the Kurdish autonomous region in northern Iraq and replace them "with a caliphate governed by Sharia law," Eurojust said in a news release.

ISIS, which calls itself the Islamic State, captured parts of Syria and Iraq for what it calls its Islamic caliphate. In Iraq, one of the forces opposing ISIS is the Peshmerga, the Kurdish troops who defend the Kurdish autonomous region in Iraq.

On Thursday, the Peshmerga, backed by air support from a U.S.-led coalition, launched an offensive to take back the Iraqi town of Sinjar from ISIS.